It Was Fast on My Machine: What Production Reveals About Web Performance

Your application feels instant on localhost but slows down in production. This common gap between development and real environments is a major, often misunderstood source of performance issues. This talk explains why “fast locally” becomes “slow in production,” showcasing post-deployment factors like caching behaviour, HTTP headers, CDN interactions, runtime differences, and missing observability.

Using concrete examples from real systems, the talk will demonstrate how to think about end-to-end latency, validate assumptions with data, and design applications that behave predictably at scale and under production traffic.

Attendees will walk away with clear insights into:

• How caching layers, headers, and CDNs are misunderstood and misconfigured

• Key differences between local environments and production runtimes

• Why latency issues stay invisible without observability

• How small configuration choices create outsized performance impact

• Thinking production-first from day one